malloc: don't compare pointers to 0

0 is not a pointer. So do not compare pointers to 0.

Do not return 0 from functions with a pointer return
type.

Problem identified with Coccinelle.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>